This invention relates to a method of improving the processing properties of isoolefinic homopolymers and copolymers, especially those commonly known as butyl rubber or isobutylene-isoprene copolymer rubber. The invention also relates to methods for producing such improved processing polymers and the specific molecular criteria which must be controlled in order to obtain these improved properties. The invention particularly relates to the achievement of improved processing properties by means of controlled and specific modification of the molecular weight distribution of these polymers. The term "butyl rubber" as used in the specification and claims means copolymers of C.sub.4 to C.sub.7 isoolefins and C.sub.4 to C.sub.14 conjugated dienes which comprise about 0.5 to about 15 mole percent conjugated diene and about 85 to 99.5 mole percent isoolefin. Illustrative examples of the isoolefins which may be used in the preparation of butyl rubber are isobutylene, 2-methyl-1-propene, 3-methyl-1-butene, 4-methyl-1-pentene and beta-pinene. Illustrative examples of conjugated dienes which may be used in the preparation of butyl rubber are isoprene, butadiene, 2,3-dimethyl butadiene, piperylene, 2,5-dimethylhexa-2,4-diene, cyclopentadiene, cyclohexadiene and methylcyclopentadiene. The preparation of butyl rubber is described in U.S. patent application No. 2,356,128 and is further described in an article by R. M. Thomas et al. in Ind. & Eng. Chem., vol. 32, pp. 1283 et seq., Oct., 1940. Butyl rubber generally has a viscosity average molecular weight between about 100,000 to about 1,500,000, preferably about 250,000 to about 800,000 and a Wijs Iodine No. (INOPO) of about 0.5 to 50, preferably 1 to 20 (for a description of the INOPO test, see Industrial and Engineering Chemistry, Vol. 17, p. 367, 1945).
The term isoolefin homopolymers as used herein is meant to encompass those homopolymers of C.sub.4 to C.sub.7 isoolefins particularly polyisobutylene, which have a small degree of terminal unsaturation and certain elastomeric properties. The principal commercial forms of these butyl rubber and isoolefin polymers such as isobutylene isoprene butyl rubber and polyisobutylene, are prepared in a low temperature cationic polymerization process using Lewis acid type catalysts, typically aluminum chloride being employed. Ethyl aluminum dichloride and boron trifluoride are also considered useful in these processes. The process extensively used in industry employs methyl chloride as the diluent for the reaction mixture at very low temperatures, that is less than minus 90.degree. C. Methyl chloride is typically employed for a variety of reasons, including the fact that it is a solvent for the monomers and aluminum chloride catalyst and a nonsolvent for the polymer product thereby resulting in a slurry. Also, methyl chloride has suitable freezing and boiling points to permit, respectively, low temperature polymerization and effective separation from the polymer and unreacted monomers. However, it is also possible to conduct such polymerizations in a diluent which is a solvent for the polymer produced, e.g., pentane, hexane and heptane and mixtures of such solvents with one another or with methyl chloride and/or methylene chloride. As will be described later, there are some advantages which can be obtained by conducting the polymerization in solution under certain conditions relating to the particular monomers and other reactants employed in the polymerization.
The slurry polymerization process in methyl chloride offers a number of additional advantages in that a polymer concentration of approximately 30% by weight in the reaction mixture can be achieved, as opposed to the concentration of only about 8% to 12% in solution polymerization. Also, an acceptable, relatively low viscosity of the polymerization mass is obtained enabling the heat of polymerization to be removed more effectively by heat exchange. Slurry polymerization processes in methyl chloride are used in the production of high molecular weight polyisobutylene and isobutylene-isoprene butyl rubber polymers.
U.S. Pat. Nos. 4,252,710, 4,358,560 and 4,474,924, each incorporated herein by reference, disclose methods for stabilizing against agglomeration the slurry polymerization product of the isoolefin homopolymers or butyl rubber copolymers polymerized in a polar chlorinated hydrocarbon diluent such as methyl chloride, methylene chloride, vinyl chloride or ethyl chloride. The significant advance of slurry stabilization disclosed in those patents is achieved by the use of a stabilizing agent being (i) a preformed copolymer having a lyophilic, polymerization diluent soluble portion and a lyophobic polymerization diluent insoluble portion, the lyophobic portion being soluble in or adsorbable by the product polymer and the stabilizing agent being capable of forming an adsorbed solubilized polymer coating around the precipitated isoolefin homopolymer or butyl copolymer to stabilize the slurry, or (ii) an in situ formed stabilizing agent copolymer formed from a stabilizer precursor, the stabilizer precursor being a lyophilic polymer containing a functional group capable of copolymerizing or forming a chemical bond with the product polymer, the functional group being cationically active halogen or cationically active unsaturation, the lyophobic portion of the stabilizing agent being product polymer, the stabilizing agent so formed being capable of forming an adsorbed solubilized polymer coating around the precipitated product polymer to stabilize the product polymer slurry.
Various classes and specific types of useful stabilizing agents are disclosed and exemplified, some of which produce substantially gel free polymers and others of which produce gelled polymers under various polymerization conditions. Some of the stabilizing agents disclosed may be useful herein under appropriate, defined conditions, but the criteria for distinguishing between those which can be used herein and those which cannot be used were not known or disclosed in those patents. In addition, in order to produce the desired polymer with improved properties as disclosed in the invention herein, the agents must be used in effective concentration in order to produce controlled amounts of defined molecular weight components in the rubber; such critical limitations were unknown in the prior art and were neither suggested nor exemplified. Furthermore, alternative means for producing the unique, improved processing polymers of this invention are available as will be further disclosed herein.
In the processing of the rubbers of commerce, it is preferred that they possess sufficient green (uncured) strength to resist excessive flow and deformation in the various handling operations. It is generally believed that green strength is related to molecular weight with green strength improving as molecular weight increases. However, it is also desirable that in certain applications such rubbers have a rapid stress relaxation rate so that the stresses imposed during forming operations relax quickly and the rubber does not slowly change its shape or pull apart due to these undissipated forming stresses. Unfortunately, stress relaxation rate is also a function of molecular weight with the relaxation rate becoming slower as molecular weight increases. Hence, as molecular weight is increased to improve green strength, stress relaxation rate is reduced. Thus, as the rubber becomes better able to resist flow and deformation in the various handling operations it becomes more prone to change shape or pull apart due to unrelaxed forming stresses and a compromise becomes necessary in which green strength is sacrificed in order to achieve sufficiently rapid stress relaxation in the particular end use application. Furthermore, increasing molecular weight in order to increase green strength can make it more difficult to process the polymer and to disperse fillers and additives.
It would be desirable to be able to alter the balance between green strength and stress relaxation rate to achieve higher green strength without sacrificing relaxation rate or faster relaxation rate without sacrificing green strength.
It has previously been disclosed that some processing properties of natural rubber can be improved by the addition of prevulcanized natural rubber latex into natural rubber (see, e.g., U.S. Pat. Nos. 1,443,149 and 1,682,857); such a product was commercialized in the early 1960's (see, e.g., W. G. Wren, Rubber Chemistry & Technology, 34,378,403 [1961]). Similarly, the addition of a gel fraction for achieving processing advantages in styrene-butadiene rubber (SBR) has also been reported (see, e.g., L. M. White, Ind. & Eng. Chem., 37, 770 [1945]); Crawford and Tiger, Ind. & Eng. Chem., 41,592 (1949). Other disclosures have also been made relating to the use of SBR crosslinked with divinyl benzene (D. L. Schoene, Ind. & Eng. Chem., 38,1246 [1946]) and blends of crosslinked latex with uncrosslinked latex (O. W. Burke, Jr., British Patent 799,043). In each of these early developments, the benefit disclosed was achieved by use of a crosslinked rubber fraction. The means of producing a controlled change in molecular weight distribution and the specific nature of the change required were not understood or disclosed, nor was the specific application of such knowledge to isoolefin homopolymers and butyl rubber polymers.
As noted above, the use of divinyl benzene to effect crosslinking in a rubber has been described. Other references have similarly disclosed the use of divinyl benzene (DVB) in a butyl polymerization process to obtain a modified product with increased resistance to cold flow (U.S. Pat. No. 2,781,334); the product is alternatively described as soluble in organic solvents and having a relatively low gel content. U.S. Pat. No. 3,135,721 is directed to a particular process which uses DVB for reducing fouling during start-up of the polymerization process and does not disclose properties for any product or distinguish between those products containing very low levels of DVB and those at higher concentrations. Other references disclose the use of DVB at even higher concentrations to produce polymers containing large amounts of gel and so are not particularly relevant to the invention herein (U.S. Pat. Nos. 2,671,774; 2,729,626; 3,548,080). Finally, a polymerization process patent (U.S. Pat. No. 3,219,641) discloses the use of DVB and high boiling equivalents in the range of 0.01 to 10 weight percent based on total polymer to clean a recycle monomer stream before drying.
Still other references disclose methods and products in which cationically polymerizable monomers such as isobutylene are grafted to halogenated polymers with reactive halogen such as chlorinated butyl rubber, polyvinylchloride, etc. (U.S. Pat. Nos. 3,476,831; 3,904,708; and 3,933,942). The objective of these references is to produce a copolymer in which a monomer such as isobutylene (or isobutylene/isoprene) is grafted onto a diene polymer backbone which may include styrene. However, the references are not directed to the butyl rubber process, the grafted materials are not essentially butyl polymers as described herein and the method of obtaining butyl polymers with the improved characteristics disclosed herein is not disclosed or suggested. Furthermore, the references are particularly limited with respect to the nature of the catalyst which can accomplish the grafting process, and ethyl aluminum dichloride, a useful catalyst herein, is described as particularly unsuitable for the purposes of these references (see, e.g., U.S. Pat. No. 3,904,708, Example 16).